Measurement method and apparatus, exposure apparatus, and device manufacturing method

ABSTRACT

A measurement method for measuring a wavefront aberration of a target optical system using an interference pattern formed by a light from a first image side slit, and a light from a second image side slit, the first and second image side slits being located at an image side of the target optical system, the first image side slit having, in a shorter direction, a width equal to or smaller than a diffraction limit of the target optical system, and the second image side slit having, in a shorter direction, a width greater than the diffraction limit of the target optical system includes the steps of obtaining three or more primary wavefronts of the target optical system from different measurement directions, and calculating a wavefront aberration of the target optical system based on the three or more primary wavefronts obtained by the obtaining step.

BACKGROUND OF THE INVENTION

The present invention relates generally to a measurement method andapparatus, and particularly to a measurement method and apparatus formeasuring an optical characteristic of a projection optical system thatprojects a pattern of a reticle (mask) to a substrate.

A projection exposure apparatus has so far been employed which uses thelithography technology to manufacture fine semiconductor devices such asa semiconductor device, e.g., an IC and an LSI, an imaging device, e.g.,a CCD, a display device, e.g., a liquid crystal panel, a magnetic head.A projection exposure apparatus transfers a pattern of a reticle (mask)onto a substrate such a wafer via a projection optical system. Since theexposure apparatus is required to precisely transfer a pattern of areticle to a substrate with a specified magnification, it is importantto use a projection optical system that has an excellent imagingcharacteristic and a restrained aberration. Especially in recent years,finer processing to the semiconductor device progresses, and a patternto be transferred has become sensitive to an aberration of an opticalsystem. Accordingly, there is a demand to highly precisely measure aprojection optical system's optical characteristic (e.g., a wavefrontaberration) with the projection optical system included in an exposureapparatus. In addition, in order to improve productivity and economicefficiency, a simple, fast, and inexpensive measurement is alsoimportant.

Conventionally, a projection optical system's wavefront aberration hasbeen measured by actually exposing a reticle pattern onto a wafer, andobserving its resist image using such a means as a scanning electronmicroscope (“SEM”). This conventional measurement method has a problemin a poor reproducibility of measurement due to a difficult SEMoperation and errors in a resist application and a development.

In order to rapidly and accurately measure a projection optical system'swavefront aberration, it is desirable to use an interference method,rather than using the conventional measurement method that exposes apattern on a resist for evaluation. However, the conventionalinterference method that uses a Fizeau interferometer, a Twyman-Greeninterferometer, or the like makes an overall system's structure complex,thus implying a large-size and high-cost problem. Thus, it is difficultto mount the interferometer on an exposure apparatus, and theconventional interference method is not viable.

Therefore, an exposure apparatus is proposed that has a comparativelysimple interferometer such as a point diffraction interferometer(hereinafter called a “PDI”), a line diffraction interferometer(hereinafter called an “LDI”), and the like. For example, see JapanesePatent Application, Publication No. 2004-273748.

The wavefront measurement using the LDI obtains a pair of primarywavefronts of the projection optical system in the measurement directionthat is a direction perpendicular to a slit's longer direction, andcalculates a wavefront of the projection optical system based on thepair of primary wavefronts. The “primary wavefront,” as used herein, isa wavefront having wavefront aberration information of the targetoptical system or projection optical system only in the measurementdirection.

In measuring the first and second primary wavefronts, one major errorcause of the wavefront to be measured is a difference of a position inthe height direction (referred to as “Z position” hereinafter) of ameasurement pattern on the image side to the exposure apparatus. Morespecifically, this is an error of a 2θ component, such as a mountain(valley) in a first measurement direction, and a valley (mountain) in asecond measurement direction, in the wavefront measurement using the LDIand two primary wavefronts whose measurement directions are orthogonalto each other. Equation 1 below defines an error amount E_(2θ) of the 2θcomponent, where dz is a Z position difference of the measurementpattern on the image side between the first and second primary wavefrontmeasurements, λ is a wavelength of an exposure light source, and NA is anumerical aperture of a projection optical system:E _(2θ) =dz×(1−sqrt(1−(NA)²)/2×λ)  EQUATION 1

For example, the error amount E_(2θ) is 1.5 mλ for dz=1 nm in theexposure apparatus having λ of 193 nm and NA of 0.9. Control in order ofseveral nanometers is arduous even with an apparatus mounted with ahighly precise stage, such as an exposure apparatus. Conceivably, ameasurement error of several mλ or greater occurs in the wavefrontmeasurement using the LDI, causing the error in the wavefrontmeasurement using the LDI.

The wavefront measurement using the LDI utilizes a wavefront of thediffracted light from the slit (slit diffracted wavefront) for areference wavefront. The manufacturing errors of the slit, such as aslit width, a thickness of a light blocking part, the parallelism in thelonger direction, and the roughness of an edge cause a differencebetween slits for the slit diffracted wavefronts, and this differencetriggers the error in the wavefront measurement using the LDI.

SUMMARY OF THE INVENTION

The present invention is directed to a measurement method that uses asimple interferometer without causing an apparatus large and expensive,and can highly precisely measure an optical characteristic or an opticalcharacteristic of a target optical system.

A measurement method according to one aspect of the present inventionfor measuring a wavefront aberration of a target optical system using aninterference pattern formed by a light from a first image side slit, anda light from a second image side slit, the first and second image sideslits being located at an image side of the target optical system, thefirst image side slit having, in a shorter direction, a width equal toor smaller than a diffraction limit of the target optical system, andthe second image side slit having, in a shorter direction, a widthgreater than the diffraction limit of the target optical system includesthe steps of obtaining three or more primary wavefronts of the targetoptical system from different measurement directions, and calculating awavefront aberration of the target optical system based on the three ormore primary wavefronts of the target optical system obtained by saidobtaining step.

A further object and other characteristics of the present invention willbe made clear by the preferred embodiments described below referring toaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view for explaining a principle of a wavefront measurementusing an LDI.

FIGS. 2A and 2B are schematic plane views showing structure of an objectside measurement mark and an image side measurement mark shown in FIG.1.

FIGS. 3A and 3B are schematic plane views showing an object sidemeasurement mark and an image side measurement mark which have slits ina direction orthogonal to the slits of the object side measurement markshown in FIG. 2A and the image side measurement mark shown in FIG. 2B.

FIGS. 4A to 4C are views for explaining a method to obtain a wavefrontinformation of a target optical system from two primary wavefronts, andrespectively show a first primary wavefront, a second primary wavefront,and a target optical system's wavefront.

FIG. 5 is a schematic plane view of an image side measurement markhaving slits arranged in two directions.

FIG. 6 is a schematic plane view of an image side measurement markhaving slits arranged in three directions.

FIGS. 7A to 7C are views for explaining a method to obtain a wavefrontinformation of a target optical system from two primary wavefronts withmeasurement directions that are different by an angle of θ, andrespectively show a first primary wavefront, a second primary wavefront,and a target optical system's wavefront.

FIG. 8 is a schematic sectional view showing a structure of an exposureapparatus according to one aspect of this invention.

FIG. 9 is a schematic plane view of an object side measurement markprovided on a reticle stage shown in FIG. 8.

FIG. 10 is a schematic plane view of an image side measurement markprovided on a wafer stage shown in FIG. 8.

FIG. 11 is a schematic plane view showing an image side measurement markreplaceable with that shown in FIG. 10.

FIG. 12 is a flowchart for explaining fabrication of devices.

FIG. 13 is a detailed flowchart for a wafer process in step 4 shown inFIG. 12.

DESCRIPTION OF THE EMBODIMENTS

Referring now to the accompanying drawings, a description will be givenof a preferred embodiment of this invention. In each figure, the samereference numeral is assigned to the same component to avoid a duplicateexplanation.

Referring to FIG. 1, a description will now be given of the principle ofthe wavefront measurement using the LDI. This embodiment uses aprojection optical system 20 used for the exposure apparatus as anexample of the target optical system.

The wavefront measurement using the LDI arranges an object sidemeasurement mark 40 having a pair of parallel adjacent slits, i.e., slit42A and slit 42B, at an object side of a target optical system 20. A(slit) width in a shorter direction of at least one slit in the objectside measurement mark 40, i.e., the slit 42A in this embodiment, is madeequal to or smaller than the resolving power of the target opticalsystem 20 at the object side. As shown in FIG. 2A, the width d of theslit 42A should preferably satisfy d≦0.5×λ/na, where na is a numericalaperture of the target optical system 20 at the object side, and λ is awavelength. Here, FIG. 2A is a schematic plane view of the object sidemeasurement mark 40.

If the light from an illumination optical system 10 is used toilluminate the slits 42A and 42B, the light exiting from the slit 42Ahas an aplanatic wavefront in the shorter direction of the slit 42A. Theslit 42B may have a width equal to or wider than that of the slit 42A.When the slit 42B has a width wider than the resolving power of thetarget optical system 20 at the object side, the light having awavefront influenced by the aberration of the illumination opticalsystem 10 is emitted from the slit 42B.

A width in the longer direction of each of the slits 42A and 42B shouldbe made narrower than the so-called isoplanatic region, in which theprojection optical system 20's aberration is regarded to be identical.In addition, the slits 42A and 42B are located adjacently so that thedistance between them is narrower than the isoplanatic region.

The lights exiting from the slits 42A and 42B pass the projectionoptical system 20, and their wavefronts are influenced by the projectionoptical system 20's aberration, imaging the slits 42A and 42B on theimage plane of the projection optical system 20.

An image side measurement mark 50 is located on the image side of theprojection optical system 20 so that a slit 52C is located at the imageposition of the slit 42A, and a slit 52D at the image position of theslit 42B. The slit 52D's (slit) width D in the shorter direction is setequal to or smaller than the resolving power of the projection opticalsystem 20 on the image side. As shown in FIG. 2B, the width D of theslit 52D preferably satisfies D≦0.5×λ/NA, where NA is a numericalaperture of the projection optical system 20 on the object side, and λis the wavelength. FIG. 2B is a schematic plane view showing the imageside measurement mark 50.

A light imaged on the slit 52D has a wavefront affected by an aberrationof the projection optical system 20 and also affected by an aberrationof an illumination optical system 10 depending on the width of the slit42B. However, when passing through the slit 52D it has an aplanaticwavefront in the shorter direction of the slit 52D.

On the other hand, the (slit) width of the slit 52C in the shorterdirection is set sufficiently greater than the diffraction limit of theprojection optical system 20 or is preferably about ten to hundred timesas wide as the wavelength. The light imaged on the slit 52C has awavefront affected only by the aberration of the projection opticalsystem 20 in the shorter direction of the slit 52C. Since the slit(window) width of the slit 52C is sufficiently wide, the emitted lighthas a wavefront affected only by the aberration of the projectionoptical system 20.

The lights from the slits 52C and 52D interfere with each other, formingan interference pattern. By detecting the interference pattern using aimage sensor 30 such as a CCD, a first primary wavefront of theprojection optical system 20 is acquired with a correct relativerelationship in a measurement direction perpendicular to the longerdirection of the slit. The “primary wavefront,” as used herein, is awavefront having wavefront aberration information of the target opticalsystem or projection optical system 20 only in the measurementdirection.

As shown in FIGS. 3A and 3B, object side measurement marks 40′ and 50′having slits 42A′, 42B′, 52C′, and 52D′ in a direction orthogonal to theslits 42A, 42B, 52C and 52D are used to similarly obtain a secondprimary wavefront. FIG. 3A is a schematic plane view showing the objectside measurement mark 40′ having the slits in the direction orthogonalto the slits of the object side measurement mark 40. FIG. 3B is aschematic plane view showing the image side measurement mark 50′ havingthe slits in the direction orthogonal to the slits of the image sidemeasurement mark 50.

The wavefront of the projection optical system 20 can be obtained fromphase information in two measurement directions using a pair of primarywavefronts, i.e., the first and second primary wavefronts.

Referring now to FIGS. 4A to 4C, a description will be given of a methodfor obtaining wavefront information of the projection optical system 20from two primary wavefronts. FIGS. 4A, 4B, and 4C show a first primarywavefront, a second primary wavefront, and a projection optical system20's wavefront, respectively. The phaseal relationship in FIG. 4A on aline parallel to the Y-axis is equal to the phaseal relationship on thesame line in FIG. 4C. The phaseal relationship in FIG. 4B on a lineparallel to the X-axis is equal to the phaseal relationship on the sameline in FIG. 4C. From these relative relationships, the phase of anarbitrary point G (x, y) on a wavefront of the projection optical system20 shown FIG. 4C is calculated as phase change amounts E(0, y)−E(0, 0)and F(x, y)−F(0, y) along the arrows in FIG. 4C, and expressed in thefollowing equation:G(x,y)=E(0,y)−E(0,0)+F(x,y)−F(0,y)  EQUATION 2

The wavefront of the projection optical system 20 shown in FIG. 4C canbe thus obtained from two primary wavefronts shown in FIGS. 4A and 4B.However, as mentioned above, the thus obtained wavefront of theprojection optical system 20 contains a measurement error due to anerror of the Z position and manufacturing errors of the slits.

Accordingly, the measurement method and apparatus of this embodimentaverages the measured wavefronts, and reduces the measurement error dueto the error of the Z position of the measurement pattern andmanufacturing errors of the slits in the image side measurement marks.

FIG. 5 is a schematic plane view showing an image side measurement mark70 having slits 70A₁, 70A₂, 70B₁, and 70B₂ arranged in two directions orhaving two measurement directions, as in the prior art. In other words,the image side measurement mark 70 incorporates the slits 52C and 52Dshown in FIG. 2B with the slits 52C′ and 52D′ shown in FIG. 3B.

Referring now to FIG. 5, a description will be given of the number ofwavefronts calculated from n primary wavefronts. Assume three primarywavefronts W1(70A₁), W1(70A₂), and W1(70B₁) obtained from the slits70A₁, 70A₂, and 70B₁ for n=3. In this case, the calculated wavefrontsare W(70A₁, 70B₁) and W(70A₂, 70B₁), and the number of wavefronts N_(d2)is 2. Similarly, when n is an odd number, the number of wavefrontsN_(dn) calculated from n primary wavefronts is defined as((n−1)/2)×((n+1)/2) as expressed below:N _(d2)=(n ²−1)/4 (n: odd number)  EQUATION 3

Assume four primary wavefronts W1(70A₁), W1(70A₂), W1(70B₁), andW1(70B₂) obtained from the slits 70A₁, 70A₂, 70B₁, and 70B₂ for n=4. Inthis case, the calculated wavefronts are W(70A₁, 70B₁), W(70A₁, 70B₂),W(70A₂, 70B₁) and W(70A₂, 70B₂) and thus the number of wavefronts N_(d2)is 4 Similarly, when n is an even number, the number of wavefrontsN_(dn) calculated from n primary wavefronts is defined as (n/2)² asexpressed below:N _(d2) =n ²/4(n: even number)  EQUATION 4

FIG. 6 is a schematic plane view of the image side measurement mark 80having slits 80A, 80B, and 80C arranged in three directions or havingthree measurement directions. Referring to FIG. 6, a description will begiven of the number of wavefronts calculated from n primary wavefrontsfor n measurement directions.

Assume three primary wavefronts W1(80A), W1(80B), and W1(80C) obtainedfrom the slits 80A, 80B, and 80C for n=3. In this case, the calculatedwavefronts are W(80A, 80B), W(80A, 80C), and W(80B, 80C), and thus thenumber of wavefronts N_(d3) is 3. Similarly, when n is an odd number,the number of wavefronts N_(dn) calculated from n primary wavefronts isdefined as (n−1)+(n−2)+ . . . 1 as expressed below:N _(dn) =n(n−1)/2 (n: integer)  EQUATION 5

For n>2, N_(dn)>N_(d2). The number of wavefronts for three or moremeasurement directions calculated from the same number of primarywavefronts is more than that for two measurement directions. Thisconfiguration can increase the number of averaging operations for thesame number of measurements, and effectively reduce a measurement errordue to the Z position error and manufacturing errors of the slits.

Although the wavefront measurement using the LDI and primary wavefrontsobtained from two orthogonal measurement directions is well-known, twomeasurement directions do not have to be orthogonal to each other.Referring now to FIG. 7B, a description will be given of a method ofobtaining the wavefront information of the target optical system fromtwo primary wavefronts when the measurement directions are different byan angle of θ. FIGS. 7A, 7B, and 7C respectively show the first primarywavefront, the second primary wavefront, and the wavefronts of thetarget optical system. The phaseal relationship in FIG. 7A on a lineparallel to the Y-axis is equal to the phaseal relationship on the sameline in FIG. 7C. The phaseal relationship in FIG. 7B on a line parallelto the X′-axis is equal to the phaseal relationship on the same line inFIG. 7C. From these relative relationships, the phase of an arbitrarypoint G (x, y) on a wavefront of the target optical system shown FIG. 7Cis calculated as phase change amounts E(0, ym)−E(0, 0) and F(x, y)−F(0,ym) along the arrows in FIG. 7C, and expressed in the followingequation:G(x,y)=E(0,ym)−E(0,0)+F(x,y)−F(0,ym)  EQUATION 6

The wavefront of the target optical system shown in FIG. 4C can be thusobtained from two primary wavefronts shown in FIGS. 7A and 7B. Here, ymis a Y coordinate of an intersection between the Y-axis and a lineparallel to the X′-axis that passes a point (x, y), and expressed asfollows:ym=y−x×tan θ  EQUATION 7

While this embodiment uses the image side measurement mark 80 having theslits 80A, 80B, and 80C arranged in three directions to obtain theprimary wavefronts in three or more measurement directions, the slits donot have to be arranged in three directions. For example, the primarywavefronts in three or more measurements directions may be obtained byrotating the image side measurement mark.

A description will now be given of an exposure apparatus to which ameasurement method and apparatus of this embodiment are applied. FIG. 8is a schematic sectional view showing the structure of an exposureapparatus 300 according to one aspect of this invention. The exposureapparatus 300 includes an illumination apparatus 310, a reticle stage320 on which to locate a reticle RT, a projection optical system 330,and a wafer stage 340 on which to locate a wafer WF. The exposureapparatus further includes a image sensor 350, an object sidemeasurement mark 400, and an image side measurement mark 500, whichconstitute a measurement apparatus (interferometer).

The exposure apparatus 300 is a projection exposure apparatus thatexposes a circuit pattern of a reticle RT onto a wafer WF in astep-and-scan manner. The exposure apparatus 300 may use astep-and-repeat manner. In this embodiment, a description will be givenbelow of a step-and-scan exposure apparatus as an example.

The illumination apparatus 310, which illuminates the reticle RT onwhich a circuit pattern to be transferred is formed, includes a lightsource section (not shown), and an illumination optical system (notshown). The illumination apparatus 310 illuminates the object sidemeasurement mark 400, which will be described later.

The light source section can use as a light source, for example, an ArFexcimer laser with a wavelength of about 193 nm, a KrF excimer laserwith a wavelength of about 248 nm, and the like, and the number oflasers is not limited. The kind of a light source is not limited to theexcimer laser, and an F₂ laser with a wavelength of about 157 nm may beused.

The illumination optical system 14 is an optical system that illuminatesthe reticle RT and the object side measurement mark 400.

The reticle RT is made, for example, of quartz, on which a circuitpattern to be transferred is formed, and is supported and driven by thereticle stage 320. The diffracted lights from the reticle RT pass theprojection optical system 330, and are projected onto the wafer WF. Thereticle RT and the wafer WF are located in an optically conjugaterelationship. The exposure apparatus 300 is of a step-and-scan type, anda pattern on the reticle RT is transferred onto the wafer WF when thereticle RT and the wafer WF are scanned at a speed ratio of thereduction ratio.

The reticle stage 320 supports the reticle RT and the object sidemeasurement mark 400 via a reticle chuck (not shown), and is connectedto a transport mechanism (not shown). The transport mechanism (notshown) includes a linear motor etc., and drives the reticle stage 320 inthe X-axis direction, thus moving the reticle RT and the object sidemeasurement mark 400.

The projection optical system 330 serves to image, onto the wafer WF,the diffracted lights that have passed a pattern of the reticle RT. Theprojection optical system 330 may use a dioptric system solely includinga plurality of lens elements, a catadioptric system including aplurality of lens elements and at least one concave mirror, etc.

The wafer WF is supported and driven by the wafer stage 340. A substrateto be exposed is the wafer WF in this embodiment, but may cover a widerange of substrates such as a glass plate and other processed objects. Aphotoresist is applied to the wafer WF.

The wafer stage 340 supports the wafer WF and the image side measurementmark 500. The wafer stage 340 can use a linear motor, for example, tomove the wafer WF in the XY directions. The reticle RT and the wafer WFare synchronously scanned, for example, and a laser interferometer, etc.monitors positions of the wafer stage 340 and the reticle stage 320. Thewafer stage 340 and the reticle stage 320 are moved at a constant speedratio.

A description will now be given of a measurement apparatus or aninterferometer that measures a wavefront aberration of the projectionoptical system 330. As described above, this apparatus includes theimage sensor 350, the object side measurement mark 400, and the imageside measurement mark 500. The exposure light emitted from theillumination apparatus 310 is utilized for the light for illuminatingthe object side measurement mark 400.

FIG. 9 is a schematic plane view showing a structure of the object sidemeasurement mark 400 located on the reticle stage 320. FIG. 10 is aschematic plane view showing a structure of the image side measurementmark 500 located on the wafer stage 340. The image and object sidemeasurement marks 400 and 500 have measurement patterns for measuringthe primary wavefronts in three or more measuring directions, e.g., fourmeasurement directions in this embodiment. The coordinate system is setto that of the projection optical system 330 such that the Z-axis is theup-and-down direction of the exposure apparatus 300, the Y-axis is thedirection perpendicular to the paper plane, and the X-axis is thedirection orthogonal to the Z-axis and Y-axis. Then, on the XY plane ofthe projection optical system 330, this embodiment can measure theprimary wavefronts in the measurement directions that have angles of 0°,45°, 90°, and 135° from the X-axis.

The object side measurement mark 400 has a first measurement pattern410A, a second measurement pattern 410B, a third measurement pattern410C, and a fourth measurement pattern 410D. The image side measurementmark 500 has a first measurement pattern 510A, a second measurementpattern 510B, a third measurement pattern 510C, and a fourth measurementpattern 510D.

The first measurement pattern 410A has a pair of slits 412A and 414A fora measurement direction, for example, of 0°. The (slit) width d of theslit 412A in its shorter direction is set to about d=0.5×λ/na, where nais a numerical aperture of the projection optical system 330 on theobject side, and λ is the wavelength of the exposure light from theillumination apparatus 310. The (slit) width of the slit 414A in itsshorter direction is equal to or wider than that of the slit 412A in itsshort direction.

An image of the first measurement pattern 410A is formed onto the firstmeasurement pattern 510A of the image side measurement mark 500 via theprojection optical system 330. The first measurement pattern 510A has apair of slits 512A and 514A. The slit 512A has a (slit) width in itsshorter direction substantially greater than the diffraction limit ofthe projection optical system 330. The (slit) width D of the slit 514Ain its shorter direction D is set to about D=0.5×λA/NA, where NA is thenumerical aperture of the projection optical system 330 on the imageside, and λ is the wavelength of the exposure light from theillumination apparatus 310.

Two lights that have passed the first measurement pattern 510A interferewith each other, and form an interference pattern. This interferencepattern is taken by the image sensor 350 on the wafer stage 340,providing the first primary wavefront of the projection optical system330.

Subsequently, the reticle stage 320 is moved or the illumination opticalsystem's illumination area is changed, and the second measurementpattern 410B (having the measurement direction of 90°) orthogonal to thefirst measurement pattern 410A of the object side measurement mark 400is illuminated. Thereby, an image of the second measurement pattern 410Bis formed on the second measurement pattern 510B of the image sidemeasurement mark 500 via the projection optical system 330, thusproviding the second primary wavefront of the projection optical system330, similar to the first measurement pattern 410A.

The first primary wavefront W(0°) of the projection optical system 330is obtained from these two primary wavefronts, i.e., the first andsecond primary wavefronts.

Similar to the first primary wavefront W(0°), the second primarywavefront W(45°) is obtained. More specifically, this measurement usesthe third measurement pattern 410C having the measurement direction of45° and the fourth measurement pattern 410D orthogonal to it in theobject side measurement mark 400, and the third and fourth measurementpatterns 510C and 510D in the image side measurement mark 500. Thefourth measurement patterns 410D and 510D have the measurement directionof 135°.

In changing the illumination area of the illumination optical system toilluminate the first measurement pattern 410A to the fourth measurementpattern 410B so as to obtain the second primary wavefront W(45°) of theprojection optical system 330, the first measurement pattern 410A to thefourth measurement pattern 410B are located in the isoplanatic region.

In order to avoid a spherical aberration of a transparent plate having ameasurement pattern, the object side measurement mark 400 and the imageside measurement mark 500 are drawn on the side of the projectionoptical system 30 of the transparent plate. If the illumination opticalsystem has poor spatial coherence, a grating 370 may be inserted abovethe object side measurement mark 400 so as to illuminate the measurementpattern (aperture) of the object side measurement mark 400 with anarbitrary order diffracted light.

The final wavefront of the projection optical system 300 is calculatedby taking the 2θ component from each of the first primary wavefrontW(0°) and the second primary wavefront W(45°) of the projection opticalsystem 330. More specifically, the final wavefront of the projectionoptical system 300 is determined by using a value of the first primarywavefront W(0°) for the sin2θ component, and by using an average of thefirst primary wavefront W(0°) and the second primary wavefront W(45°)for the other components.

As described above, the cos2θ component of the first primary wavefrontW(0°) of the projection optical system 330 is a Z position differencebetween the first measurement pattern 410A (or 510A) and the secondmeasurement pattern 410B (or 510B). The sin2θ component of the secondprimary wavefront W(45°) of the projection optical system 330 is a Zposition difference between the third measurement pattern 410C (or 510C)and the fourth measurement pattern 410D (or 510D). This is because themeasurement directions of the first and second measurement patternsincline by 45° to the X-axis and Y-axis of the projection optical system330.

Since the cos2θ component is orthogonal to the sin2θ component, the Zposition error in measuring the first primary wavefront W(0°) of theprojection optical system 330 does not affect the sin2θ component of thefirst primary wavefront W(0°). Similarly, the Z position error inmeasuring the second primary wavefront W(45°) of the projection opticalsystem 330 does not affect the cos2θ component of the second primarywavefront W(45°). Thus, the obtained wavefront of the projection opticalsystem 330 is not affected by the Z position error of the measurementpattern.

The exposure apparatus 300 thus uses the simple measurement apparatus(i.e., an object side measurement mark 400, an image side measurementmark 500, and a image sensor 350), and acquires precise aberrationinformation or wavefront aberration over the pupil area in theprojection optical system 330 that serves as a target optical system.

In addition, the projection optical system 330 has a correction opticalsystem (not shown) that corrects the aberration of the projectionoptical system 330 by feeding back the measured wavefront aberration tothe projection optical system 330. The correction optical system makesmultiple optical elements (not shown) movable in and orthogonal to theoptical-axis direction, and drives one or more optical elements based onthe aberration information obtained by this embodiment. Thereby, it ispossible to correct or optimize the wavefront aberration of theprojection optical system 330. The adjusting mechanism of the aberrationof the projection optical system 330 can use various known systems, inaddition to the movable lens, such as a movable mirror (for acatadioptric or catoptric system), an inclinable plane-parallel plate, apressure controllable space, an actuator-operated plane correction, andso on.

The image side measurement mark 500 may be replaced with an image sidemeasurement mark 700 shown in FIG. 11. The image side measurement mark700 shown in FIG. 11 is different from image side measurement mark 500in an arrangement of the measurement pattern. FIG. 11 is a schematicplane view of the image side measurement mark 700.

Each of the measurement patterns 700E and 700F of the image sidemeasurement mark 700 has a united structure of the four measurementpatterns or four measurement directions of the image side measurementmarks 500. In other words, the measurement patterns 700E and 700F sharethe slit or window of the measurement patterns 510A to 510D in the imageside measurement mark 500.

The image side measurement mark 700 has measurement patterns in whicheach measurement direction is closely arranged. Therefore, the imageside measurement mark 700 can make the moving direction of the waferstage 340 shorter than that for the image side measurement mark, such asthe image side measurement mark 500, that does not share a slit ormeasurement pattern. Thereby, the image side measurement mark 700 canreduce the Z position error caused by the movement of the wafer stage340.

In exposure, the light emitted from the light source section (notshown), for example, Koehler-illuminates the reticle RT through theillumination optical system (not shown). The lights which have passedthe reticle RT and reflect a reticle pattern are imaged on the wafer WFby the projection optical system 330. As described above, the projectionoptical system 330 in the exposure apparatus 300 has a well-correctedaberration, and achieves an excellent imaging characteristic.Accordingly, the exposure apparatus 300 can provide high-definitiondevices (semiconductor devices, LCD devices, imaging devices (such asCCD, etc.), thin-film magnetic heads) with high throughput andeconomical efficiency. The simple measurement apparatus measures thewavefront aberration of the projection optical system 330 withoutincreasing a size and the cost of an apparatus.

Referring to FIGS. 12 and 13, a description will now be given of anembodiment of a device manufacturing method using the above mentionedexposure apparatus 300. FIG. 12 is a flowchart for explaining how tofabricate devices (i.e., semiconductor chips such as IC and LSI, LCDs,CCDs). Here, a description will be given of the fabrication of asemiconductor chip as an example. Step 1 (circuit design) designs asemiconductor device circuit. Step 2 (reticle fabrication) forms areticle having a designed circuit pattern. Step 3 (wafer preparation)manufactures a wafer using materials such as silicon. Step 4 (waferprocess), which is also referred to as a pretreatment, forms actualcircuitry on the wafer through the photolithography of the presentinvention using the reticle and wafer. Step 5 (assembly), which is alsoreferred to as a post-treatment, forms into a semiconductor chip thewafer formed in step 4 and includes a assembly step (e.g., dicing,bonding), a packaging step (chip sealing), and the like. Step 6(inspection) performs various tests for the semiconductor device made inStep 5, such as a validity test and a durability test. Through thesesteps, a semiconductor device is finished and shipped (Step 7).

FIG. 13 is a detailed flowchart of the wafer process in Step 4. Step 11(oxidation) oxidizes the wafer's surface. Step 12 (CVD) forms aninsulating film on the wafer's surface. Step 13 (electrode formation)forms electrodes on the wafer by vapor disposition and the like. Step 14(ion implantation) implants ion into the wafer. Step 15 (resist process)applies a photosensitive material onto the wafer. Step 16 (exposure)uses the exposure apparatus 300 to expose a circuit pattern of thereticle onto the wafer. Step 17 (development) develops the exposedwafer. Step 18 (etching) etches parts other than a developed resistimage. Step 19 (resist stripping) removes disused resist after etching.These steps are repeated, and multi-layer circuit patterns are formed onthe wafer. The device fabrication method of this embodiment maymanufacture higher quality devices than ever. Thus, the devicemanufacturing method using the exposure apparatus 300, and resultantdevices constitute one aspect of the present invention.

Furthermore, the present invention is not limited to these preferredembodiments and various variations and modifications may be made withoutdeparting from the scope of the present invention. For example, thisembodiment uses a projection optical system in the exposure apparatusfor a target optical system, but the target optical system is notlimited to the projection optical system in the exposure apparatus.

This application claims a foreign priority benefit based on JapanesePatent Application No. 2005-375946, filed on Dec. 27, 2005, which ishereby incorporated by reference herein in its entirety as if fully setforth herein.

1. A measurement method for measuring a wavefront aberration of a target optical system using an interference pattern formed by a light from a first image side slit and a light from a second image side slit, the first and second image side slits being located at an image side of the target optical system, the first image side slit having, in a shorter direction, a width equal to or smaller than a diffraction limit of the target optical system, and the second image side slit having, in a shorter direction, a width greater than the diffraction limit of the target optical system, said measurement method comprising the steps of: obtaining three or more primary wavefronts of the target optical system from different measurement directions; and calculating a wavefront aberration of the target optical system based on the three or more primary wavefronts of the target optical system obtained by said obtaining step.
 2. A measurement method according to claim 1, wherein there are four measurement directions that extend at angles of 0°, 45°, 90°, and 135° from a reference direction.
 3. A measurement method according to claim 1, wherein said obtaining step includes the steps of: obtaining a first primary wavefront of the target optical system in a first measurement direction; obtaining a second primary wavefront of the target optical system in a second measurement direction orthogonal to the first measurement direction; obtaining a third primary wavefront of the target optical system in a third measurement direction that inclines by 45° to the first measurement direction; and obtaining a fourth primary wavefront of the target optical system in a fourth measurement direction orthogonal to the third measurement direction, wherein said calculating step includes the steps of: calculating a sin2θ component of the wavefront aberration of the target optical system based on the first and second primary wavefronts; and calculating a cos2θ component of the wavefront aberration of the target optical system based on the third and fourth primary wavefronts.
 4. A measurement apparatus for measuring an optical characteristic of a target optical system, said measurement apparatus comprising: an object side measurement mark having an object side slit located at an object side of the target optical system, the object side slit having a width in a shorter direction equal to or less than a diffraction limit of the target optical system; an image side measurement mark having a first image side slit located at an image side of the target optical system, and formed in the same direction as that of the object side slit, said image side measurement mark having a first image side slit that has, in a shorter direction, a width equal to or smaller than the diffraction limit of the target optical system, and a second image side slit that has, in a shorter direction, a width greater than the diffraction limit of the target optical system; and a detector for detecting an interference pattern formed by lights from said first and second image side slits, wherein the object side slit, the first and second image side slits are configured to detect three or more primary wavefronts of the target optical system from different measurement directions.
 5. A measurement apparatus according to claim 4, wherein there are three or more object side slits having different longer directions from each other, and there are three or more pairs of first and second image side slits in the same directions as the longer directions of the three or more object side slits.
 6. A measurement apparatus according to claim 4, wherein there are four object side slits having different longer directions from each other, and there are four pairs of first and second image side slits in the same directions as the longer directions of the four object side slits, and wherein there are four measurement directions that extend at angles of 0°, 45°, 90°, and 135° from a reference direction.
 7. A measurement apparatus for measuring an optical characteristic of a target optical system, said measurement apparatus comprising: an object side measurement mark having an object side slit located at an object side of the target optical system, the object side slit having a width in a shorter direction equal to or less than a diffraction limit of the target optical system; an image side measurement mark having a first image side slit located at an image side of the target optical system, and formed in the same direction as that of the object side slit, the image side measurement mark having a first image side slit that has, in a shorter direction, a width equal to or smaller than a diffraction limit of the target optical system, and a second image side slit that has, in a shorter direction, a width greater than the diffraction limit of the target optical system; and a detector for detecting an interference pattern formed by lights from said first and second image side slits, wherein the second image side slit are configured to unite a first slit used to obtain a primary wavefront of the target optical system in a first measurement direction, with a second slit used to obtain a primary wavefront of the target optical system in a second measurement direction different from the first measurement direction.
 8. An exposure apparatus for exposing a pattern of a reticle onto a substrate using a light from a light source, said exposure apparatus comprising: a projection optical system for projecting the pattern onto said substrate; a measurement apparatus according to claim 4 for measuring an optical characteristic of the projection optical system. 